This invention relates to an improved method for suspension-polymerizing vinyl chloride. The suspension polymerization of vinyl chloride has generally been conducted in an aqueous medium containing a natural or synthetic highmolecular weight protective colloidal suspending agent, such as partially saponified polyvinyl acetate, cellulose ether, or gelatin, or a solid-dispersing agent such as calcium carbonate, magnesium carbonate, barium sulfate, titanium white or alumina, and, as a catalyst, an organic peroxide such as lauroyl peroxide, benzoyl peroxide, isopropyl peroxydicarbonate, acetyl cyclohexylsulfonyl peroxide, or an azo compound such as azobisisobutylonitrile or dimethylvaleronitrile. In carrying out the polymerization, the aqueous medium containing the above-given additives is put in a pressure-proof polymerization vessel, equipped with a stirrer and a cooling jacket, and while the temperature inside the vessel is kept between 30.degree. C. and 60.degree. C., the mixture is vigorously stirred. In mass producing the polymer, the size of the polymerization vessel, the kind of catalyst employed (which has a great bearing upon the length of the polymerization time), the method for removing heat generated by the polymerization, and the type of stirrer used present major problems which must be resolved.
The particle size and particle size distribution of the polymer prepared by the suspension polymerization of vinyl chloride greatly depend upon (i) the degree of interfacial tension between (a) the water containing the suspending agent and the emulsifier, and (b) the monomer, and (ii) the velocity at which the polymerization system is stirred in the early stages of the polymerization. Consequently many proposals have been made as to the choice of the suspending agent. Furthermore, although less vigorous stirring will suffice in the latter stages of the process, stirring is carried out at the same velocity throughout the polymerization in order to avoid the use of complicated apparatus. The polymerization temperature must be kept constant throughout the process. Therefore, the polymerization vessel is usually equipped with a cooling jacket. In the suspension polymerization of vinyl chloride, the polymerization velocity suddenly rises when a high conversion rate is reached. This is due to the insolubility of the polymer in the monomer and is called the "Tromsdorf Effect" or "Gel Effect." As a result, the polymer accumulates heat within itself, making it difficult to obtain a uniform degree of polymerization and the superior particle size distribution and porosity desired. When the polymer is to be mass produced, the polymerization vessel must be enlarged accordingly. Additionally, the stirrer with which it is equipped must be provided with increased power. As a result, there is an increase in waste of motive power in the latter stages of the polymerization process. What is worse, the surface area of the vessel does not increase proportionately with an increase in the volume of the vessel. Consequently, the area needed for heat transfer to the cooling jacket becomes insufficient. This further aggravates the above described disadvantages.
To solve the problem, a proposal has been made to equip the polymerization vessel with a condenser, which is well known in the polymerization of other vinyl monomers. It was thought that the condenser would not only cool the vessel contents but would also enhance the stirring effect. However, the installation of a condenser may cause deposition of polymer scale at the point at which the monomer comes in contact with the condenser as well as on the inside walls of the conduit pipe connecting the portion of the vessel containing the gaseous phase and the condenser. This virtually negates the function of the condenser during the course of polymerization. Furthermore, it degrades the quality of the product and endangers the operation of the apparatus. Therefore the proposal to equip the polymerization vessel with a condenser in the suspension polymerization of vinyl chloride has been rejected.